In recent years, progress has been made in increasing the strength of steel. On the other hand, the problem has arisen of a drop in the machinability. For this reason, there is a rising need for steel holding the strength while preventing a drop in the machining efficiency.
In the past, to improve the machinability of steel, there is the method of adding Pb or S as an ingredient, but Pb has the problem of environmental load. With S, there is the problem that if increasing the amount added, the mechanical properties are degraded.
Further, the fact that by the addition of Ca, the oxides in the steel are softened and are made to deposit on the tool surface during machining so as to protect the tool, the so-called “belag”, is utilized in accordance with need. However, with use of the belag, there are many limits on the machining conditions and ingredients. This is therefore not generally used.
With this as the backdrop, free cutting steels of new compositions of ingredients and machining methods of the same have been disclosed.
PLT 1 discloses steel for machine structure use which defines the ingredients of steel for machine structure use in a predetermined range so as to give an excellent machinability in a broad cutting speed region and give both high impact characteristics and a high yield ratio.
PLT 2 discloses a machining method for steel for machine structure use excellent in tool lifetime in intermittent machining which cuts steel for machine structure use of a predetermined composition of ingredients by a predetermined tool and contact time and non-contact time for steel for machine structure use by a cutting speed of 50 m/min or more so as to form a protective film mainly comprised of oxides on the tool surface.